The present invention pertains to glass bonded ceramic structures, such as glass bonded ceramic honeycomb structures, and a method of making thereof.
It is well known that cordierite is a good candidate for diesel particulate filter (DPF) applications due to its low Young's (elastic) modulus (E), good modulus of rupture strength (MOR), and favorably low coefficient of thermal expansion (CTE). Much activity has been devoted to the development of these materials for heavy duty and light duty diesel application in recent years. Cordierite bodies derive their low thermal expansion characteristics from the micro-cracks present in its microstructure. The thermal expansion coefficient becomes negative when these micro-cracks close up, giving cordierite a very low thermal expansion over a broad temperature range. However, the presence of micro-cracks correspondingly reduces the strength of cordierite, which makes it prone to cracking during the periodic filter regenerations that are needed to remove trapped soot from the filters by controlled combustion. Cracking is the prime cause of failure in cordierite-based DPFs. In general, ceramics with micro-cracked microstructures have limited strength, and also have the potential for thermal growth after thermal recycles that will further reduce the strength of body.
Glass-bonded ceramics comprise one of the approaches to forming strong ceramic bodies. U.S. patent application Ser. No. US 20050115214 A1, for example, discloses ceramics incorporating glass phases that are precipitates from mullite precursors, such as clays that contain iron and magnesium impurities. The glass phase is distributed on at least a portion of the acicular mullite grains at the mullite grain surfaces and at intersecting grain surfaces. These glass phases reportedly strengthen the mullite body, which makes mullite strong enough to survive in the diesel exhaust system. However, since the glass phase is not originally designed in the composition, its quantity and distribution are not well controlled.
In addition, the bonding between the mullite grains and the glass phase is sensitive to the mullite firing history. A special firing cycle for forming the glass phase becomes necessary. The lack of control over such glass phases may also affect the pore structure of mullite; thus the product structure cannot be reliably reproduced.
U.S. patent application Ser. No. 20050239640 A1 proposes strengthening porous cordierite bodies via a glass phase containing boron oxide. The method of forming the glass phase is to expose a porous body to a source of boron and then to heat the body in an oxygen containing atmosphere. The strength increases because of the fusion of ceramic grains with the oxide glass phase containing boron. However, formation of the glass phase is an intricate and prolonged process. Further, the evaporation of boron at high temperature could lead to loss of the glass phase during diesel regeneration thereby weakening the structure and leading to early failure. The process is primarily suited to oxide ceramics; controlling the oxidation of non-oxide ceramics, such as SiC, is not deemed practical.
Many known processes in this field rely on forming a glass phase in-situ in the ceramic matrix during thermal processing. The resulting glass phase is not well controlled because of the dependence on non-equilibrium thermodynamic processes. Further, the process times tend to be long. The porosity of the resultant ceramic body may be variable and is not under control.